Toxoplasma gondii is a worldwide parasite facultatively heteroxenous with intermediate hosts that are probably all warm-blooded animals, and definitive hosts, which are members of the family Felidae (Dubey and Beattie, 1988).
There are three infectious stages in the life cycle of T. gondii. Tachyzoites that are the active replicative form, bradyzoites contained in tissue cysts and sporozoites contained in sporulated oocysts. All three stages are infectious for both intermediate and definitive hosts, which may acquire a T. gondii infection mainly via ingestion of sporulated oocysts containing sporozoites, and tissue cysts containing bradyzoites or by transplacental passage of tachyzoites.
Toxoplasmosis is one of the most common zoonoses worldwide. We can easily assume that a vast proportion of human beings is infected, but the majority of T. gondii infections are asymptomatic or induce transitory and mild symptoms characterized by lymphadenopathy (Dubey and Beattie, 1988). The prevalence of toxoplasmosis is, in fact, not correlated to clinical cases, which are confined to particular cohorts of individuals at high risk. Particular risk factors are characterized by immunosuppression and pregnancy.
Congenital toxoplasmosis may cause abortion, neonatal death or foetal abnormalities with detrimental consequences for the foetus (Remington and Desmonts, 1990). The gravity of symptoms is dependent on several parameters, but particular importance is played by the period of gestation when infection occurs. If infection occurs in the third trimester, it is usually asymptomatic. In neonatal humans infection can be accompanied by a plethora of symptoms, ranging from the most common encephalomyelitis, retinochoroiditis, intracranial calcification and hydrocephalus to the non-specific symptoms of acute infection such as convulsion, splenomegaly, hepatomegaly, fever, anaemia, jaundice and lymphadenopathy.
Immunocompromised individuals represent the group at highest risk of developing symptomatic toxoplasmosis, which can occur as a new infection or a reactivation of latent infection.
Among immunocompromised persons, those infected with HIV are particularly at risk because of the reduction of immunity due to the virus, as previously described.
It has been observed that the development of encephalitis due to T. gondii infection can occur in up to 40 percent of HIV infected patients (Montoya and Remington, 2000). In immunocompromised patients, in contrast to immunocompetent patients, the course of disease can frequently be fatal if not recognized and treated early. This is due to the fact that the drugs routinely used are effective against the proliferative stages but they are ineffective in eradicating the encysted form of parasite (Luft et al., 1993). The most common drugs used to treat toxoplasmosis are pyrimethamine and sulfadiazine, used in combination.
According to epidemiological data, the source of infection for humans can vary greatly among different ethnic groups or geographical boundaries.
Tachyzoites are not resistant and they are able to survive only in very limited conditions. Therefore, it is generally believed that horizontal transmissions of T. gondii infections via tachyzoites are not important epidemiologically, even if biologically possible. On that account acute toxoplasmosis in humans has been associated only with consumption of unpasteurized goats milk (Sacks, Roberto and Brooks, 1982; Skinner et al., 1990), even if tachyzoites of T. gondii have been found in the milk of several intermediate hosts (Dubey and Beattie, 1988).
The main source of infection is an oocyst shed by a cat or a tissue cyst in intermediate hosts. These potential routes of infection influence any strategic plan to try to reduce risks of toxoplasmosis in a susceptible population. Among animals reared for food, T. gondii is frequently isolated in pigs, sheep and goats. Cattle are susceptible to infection but the parasite is rarely isolated in cattle tissues. Pigs are the most important animal source for toxoplasmosis. Serological and parasitological surveys indicate a very high prevalence of infection in reared animals.
Along with intermediate hosts a crucial role in the transmission of toxoplasmosis to humans is played by cats that become infected mainly by ingesting tissue cysts from musculature of other animals. Cats shed oocysts for only 1-2 weeks after primary infection and they usually acquire immune-based protection to following infections (Dubey and Frenkel, 1972, 1974), but the protection is not life-long (Dubey, 1995).
PREVENTION OF INFECTION
Due to the fact that toxoplasmosis is transmitted from animals to humans by means of oocysts in cat faeces, tissue cysts in muscles of intermediate hosts and tachyzoites in acute infected animals, the best approach to minimising the risk of contracting toxoplasmosis is the following:
avoid direct contact with cat faeces;
wash hands after handling cats or cat equipment;
wear mask and gloves when handling manure or for any soil contact;
cook all meat, especially pork;
use only pasteurized milk or milk products.